It is desired for treads for tires and especially for high-performance tires to maintain excellent handling stability (grip performance) on dry roads from the initial stage to the end of running. In other words, it is desired for them to maintain good grip performance during running, in addition to excellent initial grip performance.
Methods for improving grip performance by adding a resin with a specific softening point to a rubber composition for treads have been considered (see, for example, Patent Literature 1). In exemplary methods that have been considered, the amount of a low softening point resin, a liquid polymer, or the like is increased, or a low temperature softener is added, for the purpose of improving initial grip performance. For the purpose of achieving stable grip performance during running (hereafter, referred to as “grip performance during running”), on the other hand, a method of adding a high softening point resin to a rubber composition for treads has been considered.
In tires including a tread that contains a low softening point resin, however, grip performance during running is unfortunately lowered along with an increase in the temperature of the tread, though initial grip performance is improved.
In tires including a tread that contains a high softening point resin such as a coumarone-indene resin, on the other hand, initial grip performance is unfortunately greatly lowered though grip performance during running is ensured.
To solve these problems, a method of adding a combination of a low softening point resin and a high softening point resin to a rubber composition can be considered. However, since the combined amount of resins added to a rubber composition greatly affects the thermal properties of the whole rubber, the combined amount of resins that can be added to a rubber composition is limited. Consequently, the rubber composition containing a low softening point resin and a high softening point resin in combination shows lower initial grip performance and lower grip performance during running than when the respective resins are solely used. Thus, there is a need to develop techniques that simultaneously highly improve initial grip performance and grip performance during running while ensuring good abrasion resistance.